Dolly was part of the zoo's African elephant herd for more than 30 years. She was humanely put to sleep Sunday, zoo officials said.

"Dolly spent all day Saturday outside interacting with Felix and Samson and taking naps in the sun, and there was no indication that she had any immediately grave health concerns," said Karl Kranz, the chief operating officer of the zoo. "However, on Sunday morning, she was lying down inside the barn, which was atypical for Dolly. The zoo elephant team and veterinarians were mobilized immediately, and once the team was with Dolly, it was apparent that it was time to let her go."

Officials said the elephant's health had been on a gradual decline over the past few years as she aged. They don't believe her death was related to any one particular issue.

A necropsy, which is an autopsy on an animal, will be performed, which is routine, the zoo said (story continues below photos).

Photos: Dolly over the years at Md. Zoo in Baltimore

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Maryland Zoo in Baltimore

Dolly was part of the zoo's African elephant heard for more than 30 years. She was humanely put to sleep Sunday, zoo officials say.

Dolly was believed to be 38-40 years old. There is no documentation of her birth year. The elephant was brought to the U.S. from Mozambique in the late 1970s by a private owner, and she was brought to the zoo in 1982.

Officials said even as she slowed with age, she was always bright and very interested in her surroundings, including zoo visitors.

"Dolly was a magnificent elephant and very distinct from the others with her exceptionally long hair. All of the elephants have an impact on our guests. Meeting them up close and personal makes people appreciate elephants in a way that no other experience could. Dolly was a big part of that experience for thousands of people over the years," Kranz said.

The median life expectancy for female African elephants is 37.9 years, according to experts.

The zoo now has four elephants, females Anna and Felix, young male Samson and the adult bull, Tuffy.

A Justice Department civil rights investigation has concluded that the Ferguson Police Department and the city's municipal court engaged in a "pattern and practice" of discrimination against African Americans.